Problem: Which of these numbers is prime? ${12,\ 37,\ 65,\ 77,\ 85}$
A prime number has exactly two factors: 1 and itself. The factors of 12 are 1, 2, 3, 4, 6, and 12. The factors of 37 are 1 and 37. The factors of 65 are 1, 5, 13, and 65. The factors of 77 are 1, 7, 11, and 77. The factors of 85 are 1, 5, 17, and 85. Thus, 37 is a prime number.